An image forming apparatus includes a plurality of formation units which are aligned along a sheet transport belt, and sequentially forms each color image on a sheet transported on the belt by each formation unit. In such an image forming apparatus, a technique called registration is employed to reduce or prevent deviation (positional deviation) of an image forming position of each color on the sheet between the formation units.
The image forming apparatus which employs this technique includes an optical sensor having a light emitting section and a light receiving section. The light emitting section emits light to the belt, and the light receiving section receives the reflected light and outputs a light receiving signal corresponding to the amount of received light. Moreover, when performing the registration, a mark is formed on the belt by each formation unit. Then, the position of the mark is determined by reading the variation of reflectance (amount of reflected light) between the belt surface and the mark surface based on the light receiving signal from the light receiving section, and deviation of the image forming position is corrected based on the determination result.
However, in the image forming apparatus, a relative deviation angle could occur between the alignment direction of the plurality of formation units and the transport direction of the belt. This deviation angle would cause a problem which cannot be corrected by the above-described registration technique. However, measures against the deviation angle were not sufficiently studied in the related-art image forming apparatus.